Listen to your body, run offroad where possible and take the occasional rest day or easy week. Do as much as possible within these contraints - for me somewhere in the region of 80-90 a week seems to be the optimal balance of getting fit enough and avoiding injury. It would be more if those buggers at work didn't require me to turn up in return for a salary....
Running rewards "time at the crease". If you throw enough time at it the rewards will come.